WIP: broken merge but need a git checkpoint

7 files changed, 1291 insertions, 52 deletions

diff --git a/src/libcmd/command.cc b/src/libcmd/command.cc
index f28cfe5de..14bb27936 100644
--- a/src/libcmd/command.cc
+++ b/src/libcmd/command.cc
@@ -86,6 +86,11 @@ ref<Store> CopyCommand::getDstStore()
 
 EvalCommand::EvalCommand()
 {
+    addFlag({
+        .longName = "debugger",
+        .description = "start an interactive environment if evaluation fails",
+        .handler = {&startReplOnEvalErrors, true},
+    });
 }
 
 EvalCommand::~EvalCommand()
@@ -103,7 +108,7 @@ ref<Store> EvalCommand::getEvalStore()
 
 ref<EvalState> EvalCommand::getEvalState()
 {
-    if (!evalState)
+    if (!evalState) {
         evalState =
             #if HAVE_BOEHMGC
             std::allocate_shared<EvalState>(traceable_allocator<EvalState>(),
@@ -113,6 +118,11 @@ ref<EvalState> EvalCommand::getEvalState()
                 searchPath, getEvalStore(), getStore())
             #endif
             ;
+
+        if (startReplOnEvalErrors) {
+            evalState->debugRepl = &runRepl;
+        };
+    }
     return ref<EvalState>(evalState);
 }
 
diff --git a/src/libcmd/command.hh b/src/libcmd/command.hh
index 078e2a2ce..3b4b40981 100644
--- a/src/libcmd/command.hh
+++ b/src/libcmd/command.hh
@@ -57,6 +57,9 @@ struct CopyCommand : virtual StoreCommand
 
 struct EvalCommand : virtual StoreCommand, MixEvalArgs
 {
+    bool startReplOnEvalErrors = false;
+    bool ignoreExceptionsDuringTry = false;
+
     EvalCommand();
 
     ~EvalCommand();
@@ -75,10 +78,16 @@ struct MixFlakeOptions : virtual Args, EvalCommand
 {
     flake::LockFlags lockFlags;
 
+    std::optional<std::string> needsFlakeInputCompletion = {};
+
     MixFlakeOptions();
 
-    virtual std::optional<FlakeRef> getFlakeRefForCompletion()
+    virtual std::vector<std::string> getFlakesForCompletion()
     { return {}; }
+
+    void completeFlakeInput(std::string_view prefix);
+
+    void completionHook() override;
 };
 
 struct SourceExprCommand : virtual Args, MixFlakeOptions
@@ -114,12 +123,13 @@ struct InstallablesCommand : virtual Args, SourceExprCommand
     InstallablesCommand();
 
     void prepare() override;
+    Installables load();
 
     virtual bool useDefaultInstallables() { return true; }
 
-    std::optional<FlakeRef> getFlakeRefForCompletion() override;
+    std::vector<std::string> getFlakesForCompletion() override;
 
-private:
+protected:
 
     std::vector<std::string> _installables;
 };
@@ -133,9 +143,9 @@ struct InstallableCommand : virtual Args, SourceExprCommand
 
     void prepare() override;
 
-    std::optional<FlakeRef> getFlakeRefForCompletion() override
+    std::vector<std::string> getFlakesForCompletion() override
     {
-        return parseFlakeRefWithFragment(_installable, absPath(".")).first;
+        return {_installable};
     }
 
 private:
@@ -270,4 +280,8 @@ void printClosureDiff(
     const StorePath & afterPath,
     std::string_view indent);
 
+
+void runRepl(
+    ref<EvalState> evalState,
+    const ValMap & extraEnv);
 }
diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 9f5eec2b5..00a7a33b7 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -23,17 +23,6 @@
 
 namespace nix {
 
-void completeFlakeInputPath(
-    ref<EvalState> evalState,
-    const FlakeRef & flakeRef,
-    std::string_view prefix)
-{
-    auto flake = flake::getFlake(*evalState, flakeRef, true);
-    for (auto & input : flake.inputs)
-        if (hasPrefix(input.first, prefix))
-            completions->add(input.first);
-}
-
 MixFlakeOptions::MixFlakeOptions()
 {
     auto category = "Common flake-related options";
@@ -86,8 +75,7 @@ MixFlakeOptions::MixFlakeOptions()
             lockFlags.inputUpdates.insert(flake::parseInputPath(s));
         }},
         .completer = {[&](size_t, std::string_view prefix) {
-            if (auto flakeRef = getFlakeRefForCompletion())
-                completeFlakeInputPath(getEvalState(), *flakeRef, prefix);
+            needsFlakeInputCompletion = {std::string(prefix)};
         }}
     });
 
@@ -103,12 +91,10 @@ MixFlakeOptions::MixFlakeOptions()
                 parseFlakeRef(flakeRef, absPath("."), true));
         }},
         .completer = {[&](size_t n, std::string_view prefix) {
-            if (n == 0) {
-                if (auto flakeRef = getFlakeRefForCompletion())
-                    completeFlakeInputPath(getEvalState(), *flakeRef, prefix);
-            } else if (n == 1) {
+            if (n == 0)
+                needsFlakeInputCompletion = {std::string(prefix)};
+            else if (n == 1)
                 completeFlakeRef(getEvalState()->store, prefix);
-            }
         }}
     });
 
@@ -139,6 +125,24 @@ MixFlakeOptions::MixFlakeOptions()
     });
 }
 
+void MixFlakeOptions::completeFlakeInput(std::string_view prefix)
+{
+    auto evalState = getEvalState();
+    for (auto & flakeRefS : getFlakesForCompletion()) {
+        auto flakeRef = parseFlakeRefWithFragment(expandTilde(flakeRefS), absPath(".")).first;
+        auto flake = flake::getFlake(*evalState, flakeRef, true);
+        for (auto & input : flake.inputs)
+            if (hasPrefix(input.first, prefix))
+                completions->add(input.first);
+    }
+}
+
+void MixFlakeOptions::completionHook()
+{
+    if (auto & prefix = needsFlakeInputCompletion)
+        completeFlakeInput(*prefix);
+}
+
 SourceExprCommand::SourceExprCommand(bool supportReadOnlyMode)
 {
     addFlag({
@@ -146,7 +150,8 @@ SourceExprCommand::SourceExprCommand(bool supportReadOnlyMode)
         .shortName = 'f',
         .description =
             "Interpret installables as attribute paths relative to the Nix expression stored in *file*. "
-            "If *file* is the character -, then a Nix expression will be read from standard input.",
+            "If *file* is the character -, then a Nix expression will be read from standard input. "
+            "Implies `--impure`.",
         .category = installablesCategory,
         .labels = {"file"},
         .handler = {&file},
@@ -440,10 +445,8 @@ DerivedPaths InstallableValue::toDerivedPaths()
 
     // Group by derivation, helps with .all in particular
     for (auto & drv : toDerivations()) {
-        auto outputName = drv.outputName;
-        if (outputName == "")
-            throw Error("derivation '%s' lacks an 'outputName' attribute", state->store->printStorePath(drv.drvPath));
-        drvsToOutputs[drv.drvPath].insert(outputName);
+        for (auto & outputName : drv.outputsToInstall)
+            drvsToOutputs[drv.drvPath].insert(outputName);
         drvsToCopy.insert(drv.drvPath);
     }
 
@@ -466,9 +469,19 @@ struct InstallableAttrPath : InstallableValue
     SourceExprCommand & cmd;
     RootValue v;
     std::string attrPath;
-
-    InstallableAttrPath(ref<EvalState> state, SourceExprCommand & cmd, Value * v, const std::string & attrPath)
-        : InstallableValue(state), cmd(cmd), v(allocRootValue(v)), attrPath(attrPath)
+    OutputsSpec outputsSpec;
+
+    InstallableAttrPath(
+        ref<EvalState> state,
+        SourceExprCommand & cmd,
+        Value * v,
+        const std::string & attrPath,
+        OutputsSpec outputsSpec)
+        : InstallableValue(state)
+        , cmd(cmd)
+        , v(allocRootValue(v))
+        , attrPath(attrPath)
+        , outputsSpec(std::move(outputsSpec))
     { }
 
     std::string what() const override { return attrPath; }
@@ -497,7 +510,19 @@ std::vector<InstallableValue::DerivationInfo> InstallableAttrPath::toDerivations
         auto drvPath = drvInfo.queryDrvPath();
         if (!drvPath)
             throw Error("'%s' is not a derivation", what());
-        res.push_back({ *drvPath, drvInfo.queryOutputName() });
+
+        std::set<std::string> outputsToInstall;
+
+        if (auto outputNames = std::get_if<OutputNames>(&outputsSpec))
+            outputsToInstall = *outputNames;
+        else
+            for (auto & output : drvInfo.queryOutputs(false, std::get_if<DefaultOutputs>(&outputsSpec)))
+                outputsToInstall.insert(output.first);
+
+        res.push_back(DerivationInfo {
+            .drvPath = *drvPath,
+            .outputsToInstall = std::move(outputsToInstall)
+        });
     }
 
     return res;
@@ -574,6 +599,7 @@ InstallableFlake::InstallableFlake(
     ref<EvalState> state,
     FlakeRef && flakeRef,
     std::string_view fragment,
+    OutputsSpec outputsSpec,
     Strings attrPaths,
     Strings prefixes,
     const flake::LockFlags & lockFlags)
@@ -581,6 +607,7 @@ InstallableFlake::InstallableFlake(
       flakeRef(flakeRef),
       attrPaths(fragment == "" ? attrPaths : Strings{(std::string) fragment}),
       prefixes(fragment == "" ? Strings{} : prefixes),
+      outputsSpec(std::move(outputsSpec)),
       lockFlags(lockFlags)
 {
     if (cmd && cmd->getAutoArgs(*state)->size())
@@ -589,6 +616,8 @@ InstallableFlake::InstallableFlake(
 
 std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableFlake::toDerivation()
 {
+    Activity act(*logger, lvlTalkative, actUnknown, fmt("evaluating derivation '%s'", what()));
+
     auto attr = getCursor(*state);
 
     auto attrPath = attr->getAttrPathStr();
@@ -598,9 +627,41 @@ std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableF
 
     auto drvPath = attr->forceDerivation();
 
+    std::set<std::string> outputsToInstall;
+    std::optional<NixInt> priority;
+
+    if (auto aOutputSpecified = attr->maybeGetAttr(state->sOutputSpecified)) {
+        if (aOutputSpecified->getBool()) {
+            if (auto aOutputName = attr->maybeGetAttr("outputName"))
+                outputsToInstall = { aOutputName->getString() };
+        }
+    }
+
+    else if (auto aMeta = attr->maybeGetAttr(state->sMeta)) {
+        if (auto aOutputsToInstall = aMeta->maybeGetAttr("outputsToInstall"))
+            for (auto & s : aOutputsToInstall->getListOfStrings())
+                outputsToInstall.insert(s);
+        if (auto aPriority = aMeta->maybeGetAttr("priority"))
+            priority = aPriority->getInt();
+    }
+
+    if (outputsToInstall.empty() || std::get_if<AllOutputs>(&outputsSpec)) {
+        outputsToInstall.clear();
+        if (auto aOutputs = attr->maybeGetAttr(state->sOutputs))
+            for (auto & s : aOutputs->getListOfStrings())
+                outputsToInstall.insert(s);
+    }
+
+    if (outputsToInstall.empty())
+        outputsToInstall.insert("out");
+
+    if (auto outputNames = std::get_if<OutputNames>(&outputsSpec))
+        outputsToInstall = *outputNames;
+
     auto drvInfo = DerivationInfo {
-        std::move(drvPath),
-        attr->getAttr(state->sOutputName)->getString()
+        .drvPath = std::move(drvPath),
+        .outputsToInstall = std::move(outputsToInstall),
+        .priority = priority,
     };
 
     return {attrPath, getLockedFlake()->flake.lockedRef, std::move(drvInfo)};
@@ -723,8 +784,14 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
             state->eval(e, *vFile);
         }
 
-        for (auto & s : ss)
-            result.push_back(std::make_shared<InstallableAttrPath>(state, *this, vFile, s == "." ? "" : s));
+        for (auto & s : ss) {
+            auto [prefix, outputsSpec] = parseOutputsSpec(s);
+            result.push_back(
+                std::make_shared<InstallableAttrPath>(
+                    state, *this, vFile,
+                    prefix == "." ? "" : prefix,
+                    outputsSpec));
+        }
 
     } else {
 
@@ -743,12 +810,13 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
             }
 
             try {
-                auto [flakeRef, fragment] = parseFlakeRefWithFragment(s, absPath("."));
+                auto [flakeRef, fragment, outputsSpec] = parseFlakeRefWithFragmentAndOutputsSpec(s, absPath("."));
                 result.push_back(std::make_shared<InstallableFlake>(
                         this,
                         getEvalState(),
                         std::move(flakeRef),
                         fragment,
+                        outputsSpec,
                         getDefaultFlakeAttrPaths(),
                         getDefaultFlakeAttrPathPrefixes(),
                         lockFlags));
@@ -822,12 +890,13 @@ std::vector<std::pair<std::shared_ptr<Installable>, BuiltPath>> Installable::bui
                         auto outputHashes = staticOutputHashes(*evalStore, drv); // FIXME: expensive
                         auto drvOutputs = drv.outputsAndOptPaths(*store);
                         for (auto & output : bfd.outputs) {
-                            if (!outputHashes.count(output))
+                            auto outputHash = get(outputHashes, output);
+                            if (!outputHash)
                                 throw Error(
                                     "the derivation '%s' doesn't have an output named '%s'",
                                     store->printStorePath(bfd.drvPath), output);
                             if (settings.isExperimentalFeatureEnabled(Xp::CaDerivations)) {
-                                DrvOutput outputId { outputHashes.at(output), output };
+                                DrvOutput outputId { *outputHash, output };
                                 auto realisation = store->queryRealisation(outputId);
                                 if (!realisation)
                                     throw Error(
@@ -838,10 +907,11 @@ std::vector<std::pair<std::shared_ptr<Installable>, BuiltPath>> Installable::bui
                             } else {
                                 // If ca-derivations isn't enabled, assume that
                                 // the output path is statically known.
-                                assert(drvOutputs.count(output));
-                                assert(drvOutputs.at(output).second);
+                                auto drvOutput = get(drvOutputs, output);
+                                assert(drvOutput);
+                                assert(drvOutput->second);
                                 outputs.insert_or_assign(
-                                    output, *drvOutputs.at(output).second);
+                                    output, *drvOutput->second);
                             }
                         }
                         res.push_back({installable, BuiltPath::Built { bfd.drvPath, outputs }});
@@ -856,6 +926,9 @@ std::vector<std::pair<std::shared_ptr<Installable>, BuiltPath>> Installable::bui
         break;
 
     case Realise::Outputs: {
+        if (settings.printMissing)
+          printMissing(store, pathsToBuild, lvlInfo);
+
         for (auto & buildResult : store->buildPathsWithResults(pathsToBuild, bMode, evalStore)) {
             if (!buildResult.success())
                 buildResult.rethrow();
@@ -969,21 +1042,26 @@ InstallablesCommand::InstallablesCommand()
 
 void InstallablesCommand::prepare()
 {
+    installables = load();
+}
+
+Installables InstallablesCommand::load() {
+    Installables installables;
     if (_installables.empty() && useDefaultInstallables())
         // FIXME: commands like "nix profile install" should not have a
         // default, probably.
         _installables.push_back(".");
-    installables = parseInstallables(getStore(), _installables);
+    return parseInstallables(getStore(), _installables);
 }
 
-std::optional<FlakeRef> InstallablesCommand::getFlakeRefForCompletion()
+std::vector<std::string> InstallablesCommand::getFlakesForCompletion()
 {
     if (_installables.empty()) {
         if (useDefaultInstallables())
-            return parseFlakeRefWithFragment(".", absPath(".")).first;
+            return {"."};
         return {};
     }
-    return parseFlakeRefWithFragment(_installables.front(), absPath(".")).first;
+    return _installables;
 }
 
 InstallableCommand::InstallableCommand(bool supportReadOnlyMode)
diff --git a/src/libcmd/installables.hh b/src/libcmd/installables.hh
index de8b08525..948f78919 100644
--- a/src/libcmd/installables.hh
+++ b/src/libcmd/installables.hh
@@ -132,6 +132,8 @@ struct Installable
         const std::vector<std::shared_ptr<Installable>> & installables);
 };
 
+typedef std::vector<std::shared_ptr<Installable>> Installables;
+
 struct InstallableValue : Installable
 {
     ref<EvalState> state;
@@ -141,7 +143,8 @@ struct InstallableValue : Installable
     struct DerivationInfo
     {
         StorePath drvPath;
-        std::string outputName;
+        std::set<std::string> outputsToInstall;
+        std::optional<NixInt> priority;
     };
 
     virtual std::vector<DerivationInfo> toDerivations() = 0;
@@ -156,6 +159,7 @@ struct InstallableFlake : InstallableValue
     FlakeRef flakeRef;
     Strings attrPaths;
     Strings prefixes;
+    OutputsSpec outputsSpec;
     const flake::LockFlags & lockFlags;
     mutable std::shared_ptr<flake::LockedFlake> _lockedFlake;
 
@@ -164,6 +168,7 @@ struct InstallableFlake : InstallableValue
         ref<EvalState> state,
         FlakeRef && flakeRef,
         std::string_view fragment,
+        OutputsSpec outputsSpec,
         Strings attrPaths,
         Strings prefixes,
         const flake::LockFlags & lockFlags);
diff --git a/src/libcmd/local.mk b/src/libcmd/local.mk
index 7a2f83cc7..3a4de6bcb 100644
--- a/src/libcmd/local.mk
+++ b/src/libcmd/local.mk
@@ -6,9 +6,9 @@ libcmd_DIR := $(d)
 
 libcmd_SOURCES := $(wildcard $(d)/*.cc)
 
-libcmd_CXXFLAGS += -I src/libutil -I src/libstore -I src/libexpr -I src/libmain -I src/libfetchers
+libcmd_CXXFLAGS += -I src/libutil -I src/libstore -I src/libexpr -I src/libmain -I src/libfetchers -I src/nix
 
-libcmd_LDFLAGS += $(LOWDOWN_LIBS) -pthread
+libcmd_LDFLAGS = $(EDITLINE_LIBS) -llowdown -pthread
 
 libcmd_LIBS = libstore libutil libexpr libmain libfetchers
 
diff --git a/src/libcmd/markdown.cc b/src/libcmd/markdown.cc
index 29bb4d31e..668a07763 100644
--- a/src/libcmd/markdown.cc
+++ b/src/libcmd/markdown.cc
@@ -9,14 +9,16 @@ namespace nix {
 
 std::string renderMarkdownToTerminal(std::string_view markdown)
 {
+    int windowWidth = getWindowSize().second;
+
     struct lowdown_opts opts {
         .type = LOWDOWN_TERM,
         .maxdepth = 20,
-        .cols = std::max(getWindowSize().second, (unsigned short) 80),
+        .cols = (size_t) std::max(windowWidth - 5, 60),
         .hmargin = 0,
         .vmargin = 0,
         .feat = LOWDOWN_COMMONMARK | LOWDOWN_FENCED | LOWDOWN_DEFLIST | LOWDOWN_TABLES,
-        .oflags = 0,
+        .oflags = LOWDOWN_TERM_NOLINK,
     };
 
     auto doc = lowdown_doc_new(&opts);
diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
new file mode 100644
index 000000000..a8de6b80b
--- /dev/null
+++ b/src/libcmd/repl.cc
@@ -0,0 +1,1130 @@
+#include <iostream>
+#include <cstdlib>
+#include <cstring>
+#include <climits>
+
+#include <setjmp.h>
+
+#ifdef READLINE
+#include <readline/history.h>
+#include <readline/readline.h>
+#else
+// editline < 1.15.2 don't wrap their API for C++ usage
+// (added in https://github.com/troglobit/editline/commit/91398ceb3427b730995357e9d120539fb9bb7461).
+// This results in linker errors due to to name-mangling of editline C symbols.
+// For compatibility with these versions, we wrap the API here
+// (wrapping multiple times on newer versions is no problem).
+extern "C" {
+#include <editline.h>
+}
+#endif
+
+#include "ansicolor.hh"
+#include "shared.hh"
+#include "eval.hh"
+#include "eval-cache.hh"
+#include "eval-inline.hh"
+#include "attr-path.hh"
+#include "store-api.hh"
+#include "log-store.hh"
+#include "common-eval-args.hh"
+#include "get-drvs.hh"
+#include "derivations.hh"
+#include "globals.hh"
+#include "command.hh"
+#include "finally.hh"
+#include "markdown.hh"
+#include "local-fs-store.hh"
+#include "progress-bar.hh"
+
+#if HAVE_BOEHMGC
+#define GC_INCLUDE_NEW
+#include <gc/gc_cpp.h>
+#endif
+
+namespace nix {
+
+struct NixRepl
+    #if HAVE_BOEHMGC
+    : gc
+    #endif
+{
+    std::string curDir;
+    ref<EvalState> state;
+    Bindings * autoArgs;
+
+    size_t debugTraceIndex;
+
+    Strings loadedFiles;
+    typedef std::vector<std::pair<Value*,std::string>> AnnotatedValues;
+    std::function<AnnotatedValues()> getValues;
+
+    const static int envSize = 32768;
+    std::shared_ptr<StaticEnv> staticEnv;
+    Env * env;
+    int displ;
+    StringSet varNames;
+
+    const Path historyFile;
+
+    NixRepl(const Strings & searchPath, nix::ref<Store> store,ref<EvalState> state,
+            std::function<AnnotatedValues()> getValues);
+    ~NixRepl();
+    void mainLoop();
+    StringSet completePrefix(const std::string & prefix);
+    bool getLine(std::string & input, const std::string & prompt);
+    StorePath getDerivationPath(Value & v);
+    bool processLine(std::string line);
+
+    void loadFile(const Path & path);
+    void loadFlake(const std::string & flakeRef);
+    void initEnv();
+    void loadFiles();
+    void reloadFiles();
+    void addAttrsToScope(Value & attrs);
+    void addVarToScope(const Symbol name, Value & v);
+    Expr * parseString(std::string s);
+    void evalString(std::string s, Value & v);
+    void loadDebugTraceEnv(DebugTrace & dt);
+
+    typedef std::set<Value *> ValuesSeen;
+    std::ostream & printValue(std::ostream & str, Value & v, unsigned int maxDepth);
+    std::ostream & printValue(std::ostream & str, Value & v, unsigned int maxDepth, ValuesSeen & seen);
+};
+
+
+std::string removeWhitespace(std::string s)
+{
+    s = chomp(s);
+    size_t n = s.find_first_not_of(" \n\r\t");
+    if (n != std::string::npos) s = std::string(s, n);
+    return s;
+}
+
+
+NixRepl::NixRepl(const Strings & searchPath, nix::ref<Store> store, ref<EvalState> state,
+            std::function<NixRepl::AnnotatedValues()> getValues)
+    : state(state)
+    , debugTraceIndex(0)
+    , getValues(getValues)
+    , staticEnv(new StaticEnv(false, state->staticBaseEnv.get()))
+    , historyFile(getDataDir() + "/nix/repl-history")
+{
+    curDir = absPath(".");
+}
+
+
+NixRepl::~NixRepl()
+{
+    write_history(historyFile.c_str());
+}
+
+void runNix(Path program, const Strings & args,
+    const std::optional<std::string> & input = {})
+{
+    auto subprocessEnv = getEnv();
+    subprocessEnv["NIX_CONFIG"] = globalConfig.toKeyValue();
+
+    runProgram2(RunOptions {
+        .program = settings.nixBinDir+ "/" + program,
+        .args = args,
+        .environment = subprocessEnv,
+        .input = input,
+    });
+
+    return;
+}
+
+static NixRepl * curRepl; // ugly
+
+static char * completionCallback(char * s, int *match) {
+  auto possible = curRepl->completePrefix(s);
+  if (possible.size() == 1) {
+    *match = 1;
+    auto *res = strdup(possible.begin()->c_str() + strlen(s));
+    if (!res) throw Error("allocation failure");
+    return res;
+  } else if (possible.size() > 1) {
+    auto checkAllHaveSameAt = [&](size_t pos) {
+      auto &first = *possible.begin();
+      for (auto &p : possible) {
+        if (p.size() <= pos || p[pos] != first[pos])
+          return false;
+      }
+      return true;
+    };
+    size_t start = strlen(s);
+    size_t len = 0;
+    while (checkAllHaveSameAt(start + len)) ++len;
+    if (len > 0) {
+      *match = 1;
+      auto *res = strdup(std::string(*possible.begin(), start, len).c_str());
+      if (!res) throw Error("allocation failure");
+      return res;
+    }
+  }
+
+  *match = 0;
+  return nullptr;
+}
+
+static int listPossibleCallback(char *s, char ***avp) {
+  auto possible = curRepl->completePrefix(s);
+
+  if (possible.size() > (INT_MAX / sizeof(char*)))
+    throw Error("too many completions");
+
+  int ac = 0;
+  char **vp = nullptr;
+
+  auto check = [&](auto *p) {
+    if (!p) {
+      if (vp) {
+        while (--ac >= 0)
+          free(vp[ac]);
+        free(vp);
+      }
+      throw Error("allocation failure");
+    }
+    return p;
+  };
+
+  vp = check((char **)malloc(possible.size() * sizeof(char*)));
+
+  for (auto & p : possible)
+    vp[ac++] = check(strdup(p.c_str()));
+
+  *avp = vp;
+
+  return ac;
+}
+
+namespace {
+    // Used to communicate to NixRepl::getLine whether a signal occurred in ::readline.
+    volatile sig_atomic_t g_signal_received = 0;
+
+    void sigintHandler(int signo) {
+        g_signal_received = signo;
+    }
+}
+
+static std::ostream & showDebugTrace(std::ostream & out, const PosTable & positions, const DebugTrace & dt)
+{
+    if (dt.isError)
+        out << ANSI_RED "error: " << ANSI_NORMAL;
+    out << dt.hint.str() << "\n";
+
+    // prefer direct pos, but if noPos then try the expr.
+    auto pos = *dt.pos
+        ? *dt.pos
+        : positions[dt.expr.getPos() ? dt.expr.getPos() : noPos];
+
+    if (pos) {
+        printAtPos(pos, out);
+
+        auto loc = getCodeLines(pos);
+        if (loc.has_value()) {
+            out << "\n";
+            printCodeLines(out, "", pos, *loc);
+            out << "\n";
+        }
+    }
+
+    return out;
+}
+
+void NixRepl::mainLoop()
+{
+    std::string error = ANSI_RED "error:" ANSI_NORMAL " ";
+    notice("Welcome to Nix " + nixVersion + ". Type :? for help.\n");
+
+    loadFiles();
+
+    // Allow nix-repl specific settings in .inputrc
+    rl_readline_name = "nix-repl";
+    createDirs(dirOf(historyFile));
+#ifndef READLINE
+    el_hist_size = 1000;
+#endif
+    read_history(historyFile.c_str());
+    curRepl = this;
+#ifndef READLINE
+    rl_set_complete_func(completionCallback);
+    rl_set_list_possib_func(listPossibleCallback);
+#endif
+
+    /* Stop the progress bar because it interferes with the display of
+       the repl. */
+    stopProgressBar();
+
+    std::string input;
+
+    while (true) {
+        // When continuing input from previous lines, don't print a prompt, just align to the same
+        // number of chars as the prompt.
+        if (!getLine(input, input.empty() ? "nix-repl> " : "          ")) {
+            // ctrl-D should exit the debugger.
+            state->debugStop = false;
+            state->debugQuit = true;
+            break;
+        }
+        try {
+            if (!removeWhitespace(input).empty() && !processLine(input)) return;
+        } catch (ParseError & e) {
+            if (e.msg().find("unexpected end of file") != std::string::npos) {
+                // For parse errors on incomplete input, we continue waiting for the next line of
+                // input without clearing the input so far.
+                continue;
+            } else {
+              printMsg(lvlError, e.msg());
+            }
+        } catch (EvalError & e) {
+            // in debugger mode, an EvalError should trigger another repl session.
+            // when that session returns the exception will land here.  No need to show it again;
+            // show the error for this repl session instead.
+            if (state->debugRepl && !state->debugTraces.empty())
+                showDebugTrace(std::cout, state->positions, state->debugTraces.front());
+            else
+                printMsg(lvlError, e.msg());
+        } catch (Error & e) {
+            printMsg(lvlError, e.msg());
+        } catch (Interrupted & e) {
+            printMsg(lvlError, e.msg());
+        }
+
+        // We handled the current input fully, so we should clear it
+        // and read brand new input.
+        input.clear();
+        std::cout << std::endl;
+    }
+}
+
+
+bool NixRepl::getLine(std::string & input, const std::string & prompt)
+{
+    struct sigaction act, old;
+    sigset_t savedSignalMask, set;
+
+    auto setupSignals = [&]() {
+        act.sa_handler = sigintHandler;
+        sigfillset(&act.sa_mask);
+        act.sa_flags = 0;
+        if (sigaction(SIGINT, &act, &old))
+            throw SysError("installing handler for SIGINT");
+
+        sigemptyset(&set);
+        sigaddset(&set, SIGINT);
+        if (sigprocmask(SIG_UNBLOCK, &set, &savedSignalMask))
+            throw SysError("unblocking SIGINT");
+    };
+    auto restoreSignals = [&]() {
+        if (sigprocmask(SIG_SETMASK, &savedSignalMask, nullptr))
+            throw SysError("restoring signals");
+
+        if (sigaction(SIGINT, &old, 0))
+            throw SysError("restoring handler for SIGINT");
+    };
+
+    setupSignals();
+    Finally resetTerminal([&]() { rl_deprep_terminal(); });
+    char * s = readline(prompt.c_str());
+    Finally doFree([&]() { free(s); });
+    restoreSignals();
+
+    if (g_signal_received) {
+        g_signal_received = 0;
+        input.clear();
+        return true;
+    }
+
+    if (!s)
+      return false;
+    input += s;
+    input += '\n';
+    return true;
+}
+
+
+StringSet NixRepl::completePrefix(const std::string & prefix)
+{
+    StringSet completions;
+
+    size_t start = prefix.find_last_of(" \n\r\t(){}[]");
+    std::string prev, cur;
+    if (start == std::string::npos) {
+        prev = "";
+        cur = prefix;
+    } else {
+        prev = std::string(prefix, 0, start + 1);
+        cur = std::string(prefix, start + 1);
+    }
+
+    size_t slash, dot;
+
+    if ((slash = cur.rfind('/')) != std::string::npos) {
+        try {
+            auto dir = std::string(cur, 0, slash);
+            auto prefix2 = std::string(cur, slash + 1);
+            for (auto & entry : readDirectory(dir == "" ? "/" : dir)) {
+                if (entry.name[0] != '.' && hasPrefix(entry.name, prefix2))
+                    completions.insert(prev + dir + "/" + entry.name);
+            }
+        } catch (Error &) {
+        }
+    } else if ((dot = cur.rfind('.')) == std::string::npos) {
+        /* This is a variable name; look it up in the current scope. */
+        StringSet::iterator i = varNames.lower_bound(cur);
+        while (i != varNames.end()) {
+            if (i->substr(0, cur.size()) != cur) break;
+            completions.insert(prev + *i);
+            i++;
+        }
+    } else {
+        try {
+            /* This is an expression that should evaluate to an
+               attribute set.  Evaluate it to get the names of the
+               attributes. */
+            auto expr = cur.substr(0, dot);
+            auto cur2 = cur.substr(dot + 1);
+
+            Expr * e = parseString(expr);
+            Value v;
+            e->eval(*state, *env, v);
+            state->forceAttrs(v, noPos, "nevermind, it is ignored anyway");
+
+            for (auto & i : *v.attrs) {
+                std::string_view name = state->symbols[i.name];
+                if (name.substr(0, cur2.size()) != cur2) continue;
+                completions.insert(concatStrings(prev, expr, ".", name));
+            }
+
+        } catch (ParseError & e) {
+            // Quietly ignore parse errors.
+        } catch (EvalError & e) {
+            // Quietly ignore evaluation errors.
+        } catch (UndefinedVarError & e) {
+            // Quietly ignore undefined variable errors.
+        } catch (BadURL & e) {
+            // Quietly ignore BadURL flake-related errors.
+        }
+    }
+
+    return completions;
+}
+
+
+static bool isVarName(std::string_view s)
+{
+    if (s.size() == 0) return false;
+    char c = s[0];
+    if ((c >= '0' && c <= '9') || c == '-' || c == '\'') return false;
+    for (auto & i : s)
+        if (!((i >= 'a' && i <= 'z') ||
+              (i >= 'A' && i <= 'Z') ||
+              (i >= '0' && i <= '9') ||
+              i == '_' || i == '-' || i == '\''))
+            return false;
+    return true;
+}
+
+
+StorePath NixRepl::getDerivationPath(Value & v) {
+    auto drvInfo = getDerivation(*state, v, false);
+    if (!drvInfo)
+        throw Error("expression does not evaluate to a derivation, so I can't build it");
+    auto drvPath = drvInfo->queryDrvPath();
+    if (!drvPath)
+        throw Error("expression did not evaluate to a valid derivation (no 'drvPath' attribute)");
+    if (!state->store->isValidPath(*drvPath))
+        throw Error("expression evaluated to invalid derivation '%s'", state->store->printStorePath(*drvPath));
+    return *drvPath;
+}
+
+void NixRepl::loadDebugTraceEnv(DebugTrace & dt)
+{
+    initEnv();
+
+    auto se = state->getStaticEnv(dt.expr);
+    if (se) {
+        auto vm = mapStaticEnvBindings(state->symbols, *se.get(), dt.env);
+
+        // add staticenv vars.
+        for (auto & [name, value] : *(vm.get()))
+            addVarToScope(state->symbols.create(name), *value);
+    }
+}
+
+bool NixRepl::processLine(std::string line)
+{
+    line = trim(line);
+    if (line == "") return true;
+
+    _isInterrupted = false;
+
+    std::string command, arg;
+
+    if (line[0] == ':') {
+        size_t p = line.find_first_of(" \n\r\t");
+        command = line.substr(0, p);
+        if (p != std::string::npos) arg = removeWhitespace(line.substr(p));
+    } else {
+        arg = line;
+    }
+
+    if (command == ":?" || command == ":help") {
+        // FIXME: convert to Markdown, include in the 'nix repl' manpage.
+        std::cout
+             << "The following commands are available:\n"
+             << "\n"
+             << "  <expr>        Evaluate and print expression\n"
+             << "  <x> = <expr>  Bind expression to variable\n"
+             << "  :a <expr>     Add attributes from resulting set to scope\n"
+             << "  :b <expr>     Build a derivation\n"
+             << "  :bl <expr>    Build a derivation, creating GC roots in the working directory\n"
+             << "  :e <expr>     Open package or function in $EDITOR\n"
+             << "  :i <expr>     Build derivation, then install result into current profile\n"
+             << "  :l <path>     Load Nix expression and add it to scope\n"
+             << "  :lf <ref>     Load Nix flake and add it to scope\n"
+             << "  :p <expr>     Evaluate and print expression recursively\n"
+             << "  :q            Exit nix-repl\n"
+             << "  :r            Reload all files\n"
+             << "  :sh <expr>    Build dependencies of derivation, then start nix-shell\n"
+             << "  :t <expr>     Describe result of evaluation\n"
+             << "  :u <expr>     Build derivation, then start nix-shell\n"
+             << "  :doc <expr>   Show documentation of a builtin function\n"
+             << "  :log <expr>   Show logs for a derivation\n"
+             << "  :te [bool]    Enable, disable or toggle showing traces for errors\n"
+             ;
+        if (state->debugRepl) {
+             std::cout
+             << "\n"
+             << "        Debug mode commands\n"
+             << "  :env          Show env stack\n"
+             << "  :bt           Show trace stack\n"
+             << "  :st           Show current trace\n"
+             << "  :st <idx>     Change to another trace in the stack\n"
+             << "  :c            Go until end of program, exception, or builtins.break\n"
+             << "  :s            Go one step\n"
+             ;
+        }
+
+    }
+
+    else if (state->debugRepl && (command == ":bt" || command == ":backtrace")) {
+        for (const auto & [idx, i] : enumerate(state->debugTraces)) {
+            std::cout << "\n" << ANSI_BLUE << idx << ANSI_NORMAL << ": ";
+            showDebugTrace(std::cout, state->positions, i);
+        }
+    }
+
+    else if (state->debugRepl && (command == ":env")) {
+        for (const auto & [idx, i] : enumerate(state->debugTraces)) {
+            if (idx == debugTraceIndex) {
+                printEnvBindings(*state, i.expr, i.env);
+                break;
+            }
+        }
+    }
+
+    else if (state->debugRepl && (command == ":st")) {
+        try {
+            // change the DebugTrace index.
+            debugTraceIndex = stoi(arg);
+        } catch (...) { }
+
+        for (const auto & [idx, i] : enumerate(state->debugTraces)) {
+             if (idx == debugTraceIndex) {
+                 std::cout << "\n" << ANSI_BLUE << idx << ANSI_NORMAL << ": ";
+                 showDebugTrace(std::cout, state->positions, i);
+                 std::cout << std::endl;
+                 printEnvBindings(*state, i.expr, i.env);
+                 loadDebugTraceEnv(i);
+                 break;
+             }
+        }
+    }
+
+    else if (state->debugRepl && (command == ":s" || command == ":step")) {
+        // set flag to stop at next DebugTrace; exit repl.
+        state->debugStop = true;
+        return false;
+    }
+
+    else if (state->debugRepl && (command == ":c" || command == ":continue")) {
+        // set flag to run to next breakpoint or end of program; exit repl.
+        state->debugStop = false;
+        return false;
+    }
+
+    else if (command == ":a" || command == ":add") {
+        Value v;
+        evalString(arg, v);
+        addAttrsToScope(v);
+    }
+
+    else if (command == ":l" || command == ":load") {
+        state->resetFileCache();
+        loadFile(arg);
+    }
+
+    else if (command == ":lf" || command == ":load-flake") {
+        loadFlake(arg);
+    }
+
+    else if (command == ":r" || command == ":reload") {
+        state->resetFileCache();
+        reloadFiles();
+    }
+
+    else if (command == ":e" || command == ":edit") {
+        Value v;
+        evalString(arg, v);
+
+        const auto [file, line] = [&] () -> std::pair<std::string, uint32_t> {
+            if (v.type() == nPath || v.type() == nString) {
+                PathSet context;
+                auto filename = state->coerceToString(noPos, v, context, "while evaluating the filename to edit").toOwned();
+                state->symbols.create(filename);
+                return {filename, 0};
+            } else if (v.isLambda()) {
+                auto pos = state->positions[v.lambda.fun->pos];
+                return {pos.file, pos.line};
+            } else {
+                // assume it's a derivation
+                return findPackageFilename(*state, v, arg);
+            }
+        }();
+
+        // Open in EDITOR
+        auto args = editorFor(file, line);
+        auto editor = args.front();
+        args.pop_front();
+
+        // runProgram redirects stdout to a StringSink,
+        // using runProgram2 to allow editors to display their UI
+        runProgram2(RunOptions { .program = editor, .searchPath = true, .args = args });
+
+        // Reload right after exiting the editor
+        state->resetFileCache();
+        reloadFiles();
+    }
+
+    else if (command == ":t") {
+        Value v;
+        evalString(arg, v);
+        logger->cout(showType(v));
+    }
+
+    else if (command == ":u") {
+        Value v, f, result;
+        evalString(arg, v);
+        evalString("drv: (import <nixpkgs> {}).runCommand \"shell\" { buildInputs = [ drv ]; } \"\"", f);
+        state->callFunction(f, v, result, PosIdx());
+
+        StorePath drvPath = getDerivationPath(result);
+        runNix("nix-shell", {state->store->printStorePath(drvPath)});
+    }
+
+    else if (command == ":b" || command == ":bl" || command == ":i" || command == ":sh" || command == ":log") {
+        Value v;
+        evalString(arg, v);
+        StorePath drvPath = getDerivationPath(v);
+        Path drvPathRaw = state->store->printStorePath(drvPath);
+
+        if (command == ":b" || command == ":bl") {
+            state->store->buildPaths({DerivedPath::Built{drvPath}});
+            auto drv = state->store->readDerivation(drvPath);
+            logger->cout("\nThis derivation produced the following outputs:");
+            for (auto & [outputName, outputPath] : state->store->queryDerivationOutputMap(drvPath)) {
+                auto localStore = state->store.dynamic_pointer_cast<LocalFSStore>();
+                if (localStore && command == ":bl") {
+                    std::string symlink = "repl-result-" + outputName;
+                    localStore->addPermRoot(outputPath, absPath(symlink));
+                    logger->cout("  ./%s -> %s", symlink, state->store->printStorePath(outputPath));
+                } else {
+                    logger->cout("  %s -> %s", outputName, state->store->printStorePath(outputPath));
+                }
+            }
+        } else if (command == ":i") {
+            runNix("nix-env", {"-i", drvPathRaw});
+        } else if (command == ":log") {
+            settings.readOnlyMode = true;
+            Finally roModeReset([&]() {
+                settings.readOnlyMode = false;
+            });
+            auto subs = getDefaultSubstituters();
+
+            subs.push_front(state->store);
+
+            bool foundLog = false;
+            RunPager pager;
+            for (auto & sub : subs) {
+                auto * logSubP = dynamic_cast<LogStore *>(&*sub);
+                if (!logSubP) {
+                    printInfo("Skipped '%s' which does not support retrieving build logs", sub->getUri());
+                    continue;
+                }
+                auto & logSub = *logSubP;
+
+                auto log = logSub.getBuildLog(drvPath);
+                if (log) {
+                    printInfo("got build log for '%s' from '%s'", drvPathRaw, logSub.getUri());
+                    logger->writeToStdout(*log);
+                    foundLog = true;
+                    break;
+                }
+            }
+            if (!foundLog) throw Error("build log of '%s' is not available", drvPathRaw);
+        } else {
+            runNix("nix-shell", {drvPathRaw});
+        }
+    }
+
+    else if (command == ":p" || command == ":print") {
+        Value v;
+        evalString(arg, v);
+        printValue(std::cout, v, 1000000000) << std::endl;
+    }
+
+    else if (command == ":q" || command == ":quit") {
+        state->debugStop = false;
+        state->debugQuit = true;
+        return false;
+    }
+
+    else if (command == ":doc") {
+        Value v;
+        evalString(arg, v);
+        if (auto doc = state->getDoc(v)) {
+            std::string markdown;
+
+            if (!doc->args.empty() && doc->name) {
+                auto args = doc->args;
+                for (auto & arg : args)
+                    arg = "*" + arg + "*";
+
+                markdown +=
+                    "**Synopsis:** `builtins." + (std::string) (*doc->name) + "` "
+                    + concatStringsSep(" ", args) + "\n\n";
+            }
+
+            markdown += stripIndentation(doc->doc);
+
+            logger->cout(trim(renderMarkdownToTerminal(markdown)));
+        } else
+            throw Error("value does not have documentation");
+    }
+
+    else if (command == ":te" || command == ":trace-enable") {
+        if (arg == "false" || (arg == "" && loggerSettings.showTrace)) {
+            std::cout << "not showing error traces\n";
+            loggerSettings.showTrace = false;
+        } else if (arg == "true" || (arg == "" && !loggerSettings.showTrace)) {
+            std::cout << "showing error traces\n";
+            loggerSettings.showTrace = true;
+        } else {
+            throw Error("unexpected argument '%s' to %s", arg, command);
+        };
+    }
+
+    else if (command != "")
+        throw Error("unknown command '%1%'", command);
+
+    else {
+        size_t p = line.find('=');
+        std::string name;
+        if (p != std::string::npos &&
+            p < line.size() &&
+            line[p + 1] != '=' &&
+            isVarName(name = removeWhitespace(line.substr(0, p))))
+        {
+            Expr * e = parseString(line.substr(p + 1));
+            Value & v(*state->allocValue());
+            v.mkThunk(env, e);
+            addVarToScope(state->symbols.create(name), v);
+        } else {
+            Value v;
+            evalString(line, v);
+            printValue(std::cout, v, 1) << std::endl;
+        }
+    }
+
+    return true;
+}
+
+void NixRepl::loadFile(const Path & path)
+{
+    loadedFiles.remove(path);
+    loadedFiles.push_back(path);
+    Value v, v2;
+    state->evalFile(lookupFileArg(*state, path), v);
+    state->autoCallFunction(*autoArgs, v, v2);
+    addAttrsToScope(v2);
+}
+
+void NixRepl::loadFlake(const std::string & flakeRefS)
+{
+    if (flakeRefS.empty())
+        throw Error("cannot use ':load-flake' without a path specified. (Use '.' for the current working directory.)");
+
+    auto flakeRef = parseFlakeRef(flakeRefS, absPath("."), true);
+    if (evalSettings.pureEval && !flakeRef.input.isLocked())
+        throw Error("cannot use ':load-flake' on locked flake reference '%s' (use --impure to override)", flakeRefS);
+
+    Value v;
+
+    flake::callFlake(*state,
+        flake::lockFlake(*state, flakeRef,
+            flake::LockFlags {
+                .updateLockFile = false,
+                .useRegistries = !evalSettings.pureEval,
+                .allowMutable  = !evalSettings.pureEval,
+            }),
+        v);
+    addAttrsToScope(v);
+}
+
+
+void NixRepl::initEnv()
+{
+    env = &state->allocEnv(envSize);
+    env->up = &state->baseEnv;
+    displ = 0;
+    staticEnv->vars.clear();
+
+    varNames.clear();
+    for (auto & i : state->staticBaseEnv->vars)
+        varNames.emplace(state->symbols[i.first]);
+}
+
+
+void NixRepl::reloadFiles()
+{
+    initEnv();
+
+    loadFiles();
+}
+
+
+void NixRepl::loadFiles()
+{
+    Strings old = loadedFiles;
+    loadedFiles.clear();
+
+    for (auto & i : old) {
+        notice("Loading '%1%'...", i);
+        loadFile(i);
+    }
+
+    for (auto & [i, what] : getValues()) {
+        notice("Loading installable '%1%'...", what);
+        addAttrsToScope(*i);
+    }
+}
+
+
+void NixRepl::addAttrsToScope(Value & attrs)
+{
+    state->forceAttrs(attrs, [&]() { return attrs.determinePos(noPos); }, "while evaluating an attribute set to be merged in the global scope");
+    if (displ + attrs.attrs->size() >= envSize)
+        throw Error("environment full; cannot add more variables");
+
+    for (auto & i : *attrs.attrs) {
+        staticEnv->vars.emplace_back(i.name, displ);
+        env->values[displ++] = i.value;
+        varNames.emplace(state->symbols[i.name]);
+    }
+    staticEnv->sort();
+    staticEnv->deduplicate();
+    notice("Added %1% variables.", attrs.attrs->size());
+}
+
+
+void NixRepl::addVarToScope(const Symbol name, Value & v)
+{
+    if (displ >= envSize)
+        throw Error("environment full; cannot add more variables");
+    if (auto oldVar = staticEnv->find(name); oldVar != staticEnv->vars.end())
+        staticEnv->vars.erase(oldVar);
+    staticEnv->vars.emplace_back(name, displ);
+    staticEnv->sort();
+    env->values[displ++] = &v;
+    varNames.emplace(state->symbols[name]);
+}
+
+
+Expr * NixRepl::parseString(std::string s)
+{
+    Expr * e = state->parseExprFromString(std::move(s), curDir, staticEnv);
+    return e;
+}
+
+
+void NixRepl::evalString(std::string s, Value & v)
+{
+    Expr * e = parseString(s);
+    e->eval(*state, *env, v);
+    state->forceValue(v, [&]() { return v.determinePos(noPos); });
+}
+
+
+std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int maxDepth)
+{
+    ValuesSeen seen;
+    return printValue(str, v, maxDepth, seen);
+}
+
+
+std::ostream & printStringValue(std::ostream & str, const char * string) {
+    str << "\"";
+    for (const char * i = string; *i; i++)
+        if (*i == '\"' || *i == '\\') str << "\\" << *i;
+        else if (*i == '\n') str << "\\n";
+        else if (*i == '\r') str << "\\r";
+        else if (*i == '\t') str << "\\t";
+        else str << *i;
+    str << "\"";
+    return str;
+}
+
+
+// FIXME: lot of cut&paste from Nix's eval.cc.
+std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int maxDepth, ValuesSeen & seen)
+{
+    str.flush();
+    checkInterrupt();
+
+    state->forceValue(v, [&]() { return v.determinePos(noPos); });
+
+    switch (v.type()) {
+
+    case nInt:
+        str << ANSI_CYAN << v.integer << ANSI_NORMAL;
+        break;
+
+    case nBool:
+        str << ANSI_CYAN << (v.boolean ? "true" : "false") << ANSI_NORMAL;
+        break;
+
+    case nString:
+        str << ANSI_WARNING;
+        printStringValue(str, v.string.s);
+        str << ANSI_NORMAL;
+        break;
+
+    case nPath:
+        str << ANSI_GREEN << v.path << ANSI_NORMAL; // !!! escaping?
+        break;
+
+    case nNull:
+        str << ANSI_CYAN "null" ANSI_NORMAL;
+        break;
+
+    case nAttrs: {
+        seen.insert(&v);
+
+        bool isDrv = state->isDerivation(v);
+
+        if (isDrv) {
+            str << "«derivation ";
+            Bindings::iterator i = v.attrs->find(state->sDrvPath);
+            PathSet context;
+            if (i != v.attrs->end())
+                str << state->store->printStorePath(state->coerceToStorePath(i->pos, *i->value, context, "while evaluating the drvPath of a derivation"));
+            else
+                str << "???";
+            str << "»";
+        }
+
+        else if (maxDepth > 0) {
+            str << "{ ";
+
+            typedef std::map<std::string, Value *> Sorted;
+            Sorted sorted;
+            for (auto & i : *v.attrs)
+                sorted.emplace(state->symbols[i.name], i.value);
+
+            for (auto & i : sorted) {
+                if (isVarName(i.first))
+                    str << i.first;
+                else
+                    printStringValue(str, i.first.c_str());
+                str << " = ";
+                if (seen.count(i.second))
+                    str << "«repeated»";
+                else
+                    try {
+                        printValue(str, *i.second, maxDepth - 1, seen);
+                    } catch (AssertionError & e) {
+                        str << ANSI_RED "«error: " << e.msg() << "»" ANSI_NORMAL;
+                    }
+                str << "; ";
+            }
+
+            str << "}";
+        } else
+            str << "{ ... }";
+
+        break;
+    }
+
+    case nList:
+        seen.insert(&v);
+
+        str << "[ ";
+        if (maxDepth > 0)
+            for (auto elem : v.listItems()) {
+                if (seen.count(elem))
+                    str << "«repeated»";
+                else
+                    try {
+                        printValue(str, *elem, maxDepth - 1, seen);
+                    } catch (AssertionError & e) {
+                        str << ANSI_RED "«error: " << e.msg() << "»" ANSI_NORMAL;
+                    }
+                str << " ";
+            }
+        else
+            str << "... ";
+        str << "]";
+        break;
+
+    case nFunction:
+        if (v.isLambda()) {
+            std::ostringstream s;
+            s << state->positions[v.lambda.fun->pos];
+            str << ANSI_BLUE "«lambda @ " << filterANSIEscapes(s.str()) << "»" ANSI_NORMAL;
+        } else if (v.isPrimOp()) {
+            str << ANSI_MAGENTA "«primop»" ANSI_NORMAL;
+        } else if (v.isPrimOpApp()) {
+            str << ANSI_BLUE "«primop-app»" ANSI_NORMAL;
+        } else {
+            abort();
+        }
+        break;
+
+    case nFloat:
+        str << v.fpoint;
+        break;
+
+    default:
+        str << ANSI_RED "«unknown»" ANSI_NORMAL;
+        break;
+    }
+
+    return str;
+}
+
+void runRepl(
+    ref<EvalState>evalState,
+    const ValMap & extraEnv)
+{
+    auto getValues = [&]()->NixRepl::AnnotatedValues{
+        NixRepl::AnnotatedValues values;
+        return values;
+    };
+    const Strings & searchPath = {};
+    auto repl = std::make_unique<NixRepl>(
+            searchPath,
+            openStore(),
+            evalState,
+            getValues
+        );
+
+    repl->initEnv();
+
+    // add 'extra' vars.
+    for (auto & [name, value] : extraEnv)
+        repl->addVarToScope(repl->state->symbols.create(name), *value);
+
+    repl->mainLoop();
+}
+
+struct CmdRepl : InstallablesCommand
+{
+    CmdRepl() {
+        evalSettings.pureEval = false;
+    }
+
+    void prepare()
+    {
+        if (!settings.isExperimentalFeatureEnabled(Xp::ReplFlake) && !(file) && this->_installables.size() >= 1) {
+            warn("future versions of Nix will require using `--file` to load a file");
+            if (this->_installables.size() > 1)
+                warn("more than one input file is not currently supported");
+            auto filePath = this->_installables[0].data();
+            file = std::optional(filePath);
+            _installables.front() = _installables.back();
+            _installables.pop_back();
+        }
+        installables = InstallablesCommand::load();
+    }
+
+    std::vector<std::string> files;
+
+    Strings getDefaultFlakeAttrPaths() override
+    {
+        return {""};
+    }
+
+    bool useDefaultInstallables() override
+    {
+        return file.has_value() or expr.has_value();
+    }
+
+    bool forceImpureByDefault() override
+    {
+        return true;
+    }
+
+    std::string description() override
+    {
+        return "start an interactive environment for evaluating Nix expressions";
+    }
+
+    std::string doc() override
+    {
+        return
+          #include "repl.md"
+          ;
+    }
+
+    void run(ref<Store> store) override
+    {
+        auto state = getEvalState();
+        auto getValues = [&]()->NixRepl::AnnotatedValues{
+            auto installables = load();
+            NixRepl::AnnotatedValues values;
+            for (auto & installable: installables){
+                auto what = installable->what();
+                if (file){
+                    auto [val, pos] = installable->toValue(*state);
+                    auto what = installable->what();
+                    state->forceValue(*val, pos);
+                    auto autoArgs = getAutoArgs(*state);
+                    auto valPost = state->allocValue();
+                    state->autoCallFunction(*autoArgs, *val, *valPost);
+                    state->forceValue(*valPost, pos);
+                    values.push_back( {valPost, what });
+                } else {
+                    auto [val, pos] = installable->toValue(*state);
+                    values.push_back( {val, what} );
+                }
+            }
+            return values;
+        };
+        auto repl = std::make_unique<NixRepl>(
+            searchPath,
+            openStore(),
+            state,
+            getValues
+        );
+        repl->autoArgs = getAutoArgs(*repl->state);
+        repl->initEnv();
+        repl->mainLoop();
+    }
+};
+
+static auto rCmdRepl = registerCommand<CmdRepl>("repl");
+
+}